The general construction and operation of an ink-jet print cartridge using reticulated polyurethane foam is disclosed in U.S. Pat. No. 4,771,295 entitled "Thermal Ink Jet Pen Body Construction Having Improved Ink Storage and Feed Capacity" by Baker et al. issued Sep. 13, 1988.
The ink reservoir of such a print cartridge is vented to the atmosphere so that when ink is being expelled during operation of the cartridge, a vacuum is not created in the reservoir and the pressure of the ink behind the print head can be properly maintained by the foam at about -2" of water.
The size of the vent must be sufficiently large so that at the maximum printing speed of the cartridge such a vacuum is not produced, affecting the operation of the cartridge. On the other hand, the size of the vent must not be so large that the water in the ink evaporates too quickly, causing the cartridge to dry out and become useless before the end of its intended life.
There are other challenges as well in the design of vents for ink-jet print cartridges. The size of the vent and its construction must be such that when the print cartridge is held with the vent downward, the ink does not run out into the printer or onto the user. Further, the vent should be designed so that it is not easily clogged by debris and dried ink and is also not blocked by liquid ink drawn into the vent by the capillary effect.
Lastly, prior vent designs have been unable to withstand harsher environments than the normal office. A vent is needed for a print cartridge that will be used in a printer designed to be stacked in a home entertainment center. Such an environment is hotter, more dusty, and drier than offices and commercial facilities.
Thus, it should be apparent from the foregoing that the design and construction ink-jet print cartridges offer many challenges and trade offs. There is still a need for an approach that sufficiently vents at maximum cartridge printing speed, minimizes water loss through evaporation, avoids leakage, and withstands harsh environments.